The switch from mitosis to meiosis and sporulation in the yeast Saccharomyces cerevisiae, which is normally initiated by nutritional deprivation, can be specifically initiated by partial guanine or methionine deprivation. All these conditions led to the decrease of intracellular GTP whereas the concentrations of other nucleotides remain constant, increase or decrease, depending on the particular condition used to initiate meiosis. Involvement of other metabolites in sporulation induction has been ruled out; in particular, conditions can be found under which the cellular GTP decreases while that of S-adenosylmethionine or cyclic AMP does not change and yet the cells go through meiosis and sporulate well. Nuclear and nuclear matrix proteins where subjected to two-dimensional electrophoresis and certain proteins were found to change during meiosis. The gene for one nuclear matrix proteins was cloned from both a mouse and a yeast DNA library. The gene for glutamine synthetase was similarly obtained from these two libraries. In Bacillus subtilis partially inhibitory concentrations of ethionine caused continual sporulation in eth mutants. This effect depends on the conversion of ethionine to S-adenosylethionine and presumably involves the inhibition of a methylation reaction.